linux-kernel - Re: [PATCH v2 05/17] mm/frame-vector: Use FOLL

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Date:   Fri, 16 Oct 2020 00:54:18 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>
CC:     <kvm@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-samsung-soc@...r.kernel.org>, <linux-media@...r.kernel.org>,
        <linux-s390@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        "Kyungmin Park" <kyungmin.park@...sung.com>,
        Tomasz Figa <tfiga@...omium.org>,
        "Mauro Carvalho Chehab" <mchehab@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 05/17] mm/frame-vector: Use FOLL_LONGTERM

On 10/9/20 12:59 AM, Daniel Vetter wrote:
...
> @@ -48,40 +47,25 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>   
>   	start = untagged_addr(start);
>   
> -	mmap_read_lock(mm);
> -	locked = 1;
> -	vma = find_vma_intersection(mm, start, start + 1);
> -	if (!vma) {
> -		ret = -EFAULT;
> -		goto out;
> -	}
> -
> -	/*
> -	 * While get_vaddr_frames() could be used for transient (kernel
> -	 * controlled lifetime) pinning of memory pages all current
> -	 * users establish long term (userspace controlled lifetime)
> -	 * page pinning. Treat get_vaddr_frames() like
> -	 * get_user_pages_longterm() and disallow it for filesystem-dax
> -	 * mappings.
> -	 */
> -	if (vma_is_fsdax(vma)) {
> -		ret = -EOPNOTSUPP;
> -		goto out;
> -	}
> -
> -	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
> +	ret = pin_user_pages_fast(start, nr_frames,
> +				  FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM,
> +				  (struct page **)(vec->ptrs));
> +	if (ret > 0) {

None of the callers that we have today will accept anything less than
ret == nr_frames. And the whole partially pinned region idea turns out
to be just not useful for almost everyone, from what I recall of the gup/pup
call sites. So I wonder if we should just have get_vaddr_frames do the
cleanup here and return -EFAULT, if ret != nr_frames ?

thanks,
-- 
John Hubbard
NVIDIA